| The specific physical, chemical and biological properties of rhizosphere can influence the metal speciation, thus the bioavailability of metal. As a metal sink in soil, particulate organic matter (POM) is also an important source of heavy metal. Therefore, it’s significant to investigate the interaction between POM and metal in rhizosphere soil. In this study, Scanning electron microscope with energy dispersive X-ray (SEM-EDX), Fourier transform infrared (FTIR) spectrometer and X-ray absorption fine structure (XAFS) spectroscopy were combined to study the adsorption and desorption mechasims of Cu and Zn in rhizosphere soil and POM, and speciation transformation of heavy metal in rhizosphere soil and POM was investigated. The major results obtained are as follows:(1) Changes of rhizosphere organic matter concentration in different heavy metal polluted soil were investigated. During the flowing period of rice, pH of rhizosphere soil decreased, Dissolved organic carbon (DOC) concentration of rhizosphere soil (193.3-232.7 mg kg-1) was significantly higher than non-rhizosphere soil (176.7-192.5 mg kg-1). POM content of rhizosphere soil (19.6-35.0 mg g-1) was less than non-rhizosphere (22.7-44.6 mg g-1). With the increase of Cu/Zn concentration in soil, rhizosphere soil pH decreased and DOC concentration in rhizosphere increased, while POM content in rhizosphere soil was higher than non-rhizosphere soil.(2) The surface micro-morphology and elementary composition of rhizosphere POM were uncovered. SEM-EDX was used to study the surface microtopography and constitute of POM, which showed that POM was mainly composed of porous charcoal, plant residues, root fragments or fungal hyphae. Main elementary composition of POM was C, O, Al, Si, Ca, K, S and Fe, C and O in POM were greater than 50% and 30%, respectively. Elemental Analyzer was used to analysed total C, N and O content in POM, the result showed that total C, N and H of rhizosphere POM increased. BET analysis indicated that specific surface area of rhizosphere POM (7.6417 m2 g-1) was higher than non-rhizosphere POM (6.8645 m2 g-1).(3) The speciation transformation of heavy metal in rhizosphere soil and POM was clarified. Exchangeable Cu and Organic bound Fe in rhizosphere soil increased while Organic bound Cu/Zn in rhizosphere decreased. Meanwhile, Cu concentration in rhizosphere increased while Zn concentration in rhizosphere decreased. Cu/Zn concentrations in rhizosphere POM were significantly decreased. Cu in soil was mainly adsorbed on goethite, and the proportion of Cu-goethite in rhizosphere soil decreased. Cu bond to humic acid and Cu bond to citrate could make up the majority of Cu in POM, and the proportion of Cu-organic in rhizosphere POM also decreased.(4) The adsorption and desorption characteristics of Cu/Zn in rhizosphere POM were revealed. Adsorption of Cu/Zn to rhizosphere POM can be well described by Langmuir equation. Rhizosphere POM showed higher adsorption capacity for Cu and Zn than non-rhizosphere POM. The adsorption of Cu/Zn followed the trend of decrease metal sorption with the decrease of pH or increase of ionic strength. The FTIR-spectra of POM showed that carboxyl and hydroxyl groups were responsible for the metal adsorption. Citric acid was used to extract Cu/Zn in the rice rhizosphere POM, and the results could be well described by first-order reaction, power function and simplified Elovich. The amount of Cu/Zn desorption in rhizosphere POM were less than non-rhizosphere POM when the desorption reached equilibrium. Cu desorption ratio in rhizosphere POM was lower while Zn desorption ratio in rhizosphere POM was higher than non-rhizoshere POM. |
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